Paper coating compositions are generally a fluid suspension of pigment, such as clay and calcium carbonate with or without titanium dioxide in an aqueous medium with a binder such as soluble starch, modified soluble starch, styrene-butadiene copolymer emulsion, styrene-acrylic copolymer emulsion, and/or soluble modified protein to adhere the pigment to paper. Other functional or processing additives can be added in small amounts to impart properties such as thickening, lubricity, hydrophobicity, foam control, and/or anti-microbial properties and the like. For paper sizing, the main component is a starch solution, with or without inorganic pigments, and sometimes an emulsion binder to impart paper strength and repellency to water, especially during printing.
The hydrophilic nature of the binder, particularly the starch solutions, requires the presence of an insolubilizing material that crosslinks the binder making it hydrophobic, hydrophobicity in coated or sized papers is important to enable the paper to be processed through high-speed offset printing presses and can improve the printability characteristics of the surface of the coated paper. Common crosslinking materials are glyoxal resins and formaldehyde-donor agents such as melamine-formaldehyde, urea-melamine-formaldehyde, and partially or wholly methylated derivatives thereof.
Blocked glyoxal insolubilizers allow for the water resistance that is particularly critical in web offset printing, the most common commercial printing process, where aqueous dampening fountain solutions are employed. If natural binders, such as starch, in the coating formulations are not insolubilized, piling and poor dot definition results on printing press. Increases in press speed and consequent changes in ink chemistry and fountain solutions caused reassessment of the nature of coated paper and paperboard surfaces. Past coatings have been designed to achieve high levels of wet rub resistance, but this is no longer true. High speed printing processes require rapid acceptance of aqueous and oily fluids to obtain high quality print.
Insolubilizers are believed to react with hydroxyl (—OH) groups associated with starch or amino groups on protein. The amino group or hydroxyl group reacts with organic compounds such as aldehyde donors. This basic reaction between the aldehyde and the hydroxyl group of polymers such as starch is responsible for insolubilization.
The aldehyde group may be supplied by many donors. The selection of chemical type depends upon operating conditions, preparation and economic factors. The rate at which coating insolubilization is developed and the degree required covers a wide range depending on the end use. Most common paper coating and sizing insolubilizers are reacted glyoxal type compounds. Once reacted in the coating structure, the insolubilizer forms hemiacetal groups which crosslink the binder and increase water resistance. The reaction of polyol-carbonyl adducts provides the formulator a highly reactive molecule with controlled viscosity.
Glyoxal is a highly reactive monomer that cures quickly and has excellent insolubilizing properties, particularly with starch. The rapid reaction between glyoxal and binder, however, increases the viscosity of the coating composition thereby making processing of the coating difficult. Frequently, glyoxal-insolubilized coatings gel completely particularly in high solids formulations. Gelling can also occur in moderate or low solids formulations if they are not used promptly. Thus in situations where it is required that the viscosity remain stable for many hours, or where high-solids paper coatings are to be applied by high-speed coating techniques, a pure glyoxal system may be unsuitable.
Blocked or reacted glyoxal resins have been used to overcome some of the deficiencies associated with glyoxal. For example, U.S. Pat. No. 4,537,634 teaches urea, cyclic amide condensates, or polyol carbonyl adducts as blockers.
U.S. Pat. No. 4,695,606 teaches the use of blocked glyoxal, which can be mixed with binders such as starch, without reacting to any great degree. The reactivity of these blocked glyoxals, however, can be controlled so that they crosslink with the binder upon drying.
Glyoxal based insolubilizers provide advantageous insolubilization in a slightly alkaline coatings (7-8 pH), but performance drops off rapidly as the coating pH increases above pH 9. High pH (>9.0) is believed to deleteriously impact the glyoxal. Lowering the pH may not unblock and liberate the glyoxal for insolubilization. The reaction of glyoxal with free hydroxide to form a glycolate ion is known as the Cannizaro reaction and results in poor coating insolubility. When the pH of a size press or coating formulation exceeds approximately 8.5, glycolate ions are produced rapidly and the efficiency of the glyoxal based insolubilizer is substantially reduced.